Resistors made of resistor material supported on a substrate and having electrodes at two ends of the material are trimmed by lasers to precisely remove material between the electrodes so as to achieve the desired resistance for the resistor. By removing material, the flow of electricity is impeded, thereby increasing resistance. The trimming can involve a plunge cut (a single slice extending from one side into the material transverse to the direction between electrodes), an L-cut (two legs, one extending in from a side transverse to the direction between electrodes and one parallel to the direction), a U-cut (three legs, two extending from a side and transverse to the direction and one parallel to it), or a scan cut (repetitive overlapping cuts that remove a large portion of material).
U-cuts are desirable for some applications because they do not have the termination points of plunge and L-cuts (which in general disrupt electron flow and can permit arcing, e.g., when used in lightening arresters), are quicker than scan cuts and do not have thermal stresses associated with scan cuts. Achieving precise resistance with U-cuts has often involved operating in a repetitive start/stop/measure mode, which involves noncontinuous trimming that can result in microcracking. One method of avoiding the stopping of the laser has been to make several overlapping U-cuts, successive U-cuts having longer first and third legs and shorter second legs.